1. Field of the Invention
The present invention relates to a lithium secondary battery, and more particularly, to a lithium secondary battery having excellent lifetime characteristics and charge characteristics at a high voltage.
2. Description of Related Art
As the electronic, communication, and computer industries are rapidly developed, portable electronic communication devices such as camcorders, cellphones, notebook PCs, and/or the like are dramatically improved. Thus, demand for lithium second batteries as power sources to drive the above devices is increased daily. In particular, related to eco-friendly power sources of applications such as electronic vehicles, uninterruptible power devices, power tools, satellites, and/or the like, research and development is being actively progressed in Japan, Europe, the United States of America, and/or the like as well as in Korea.
Lithium secondary batteries developed in the early 1990s among currently applied secondly batteries are composed of a negative electrode formed of a carbon material, etc., which is capable of adsorbing and ejecting lithium ions, a positive electrode formed of lithium based oxides, etc., and a non-aqueous electrolyte in which lithium salts are dissolved in a proper amount of a composite organic solvent.
However, as application ranges of the lithium secondary batteries are increased, a longer lifetime is required, and as a capacity of the battery is increased, a requirement for charging at a high voltage is increased. However, when the battery is charged at the high voltage, an amount of lithium ions is greatly increased, and instability of a structure of a positive active material is greatly increased, and decomposition of the electrolyte on the surface of the positive electrode is accelerated, and thus, the lifetime of the battery is decreased. A conventional lithium transition metal oxide or composite oxide, which is used for the positive active material of the lithium secondary battery, has a limitation in the lifetime characteristics and the charging at the high voltage.
In order to solve the above problem, Korean Patent Publication No. 10-2006-0134631 disclosed core-shell structured positive electrode active materials with high capacity and safety and their preparing method for lithium secondary batteries, but the lifetime characteristics are not increased sufficiently and the charging problem at the high voltage is not solved.